how to get the gene into non-dividing cells like liver, muscle, and neurons;

how to get the gene to be replicated (in dividing cells) and expressed indefinitely but

minimize the risk that it inserts near a proto-oncogene which it could activate producing a cancer. (This occurred in several little boys treated with a retroviral vector based on the murine leukemia virus. [Link])

how to get the gene to be expressed as needed; that is, how to bring the gene under normal physiological controls so that its product is produced where, when, and in the amounts needed.

In the 1 January 1999 issue of Science, James M. Wilson and his colleagues reported the results of using this strategy in both mice and rhesus monkeys.
They injected their experimental animals with two vectors.

The experimental animals were injected (in their skeletal muscles) with many copies of both vectors. Skeletal muscle was chosen because muscle fibers are multinucleate. Once across the plasma membrane, there are many nuclei which the vectors can enter and hence many opportunities to integrate into the DNA of the host.

Later the animals were injected with rapamycin. This small molecule is an immunosuppressant and is currently being tested in transplant recipients to help them avoid rejection of the transplant. It was used here because of its ability to simultaneously bind to the FRB and FKBP12 domains of the two gene products of vector 1. The resulting trimer is an active transcription factor for the erythropoietin gene.

The results:

Both groups of animals gained control over their blood sugar level and kept this control for over 8 months. When given glucose, they proceeded to synthesize the synthetic insulin which then brought their blood glucose back down to normal levels.

Researchers at the Salk Institute have slowed up the progression of the disease in these mice by injecting their skeletal muscles with an AAV vector containing the gene for insulin-like growth factor 1 (IGF-1).
The vector

It's a big jump from mice to humans, but these results indicate that the principle of gene therapy for single-gene disorders is valid.

And some early trials in humans look promising.

An intravenous injection of an AAV vector containing the cDNA of factor IX has produced functional levels of factor IX in several men with hemophilia B.

On August 18, 2003, physicians in New York injected 3.5 x 109 copies of an AAV vector carrying a gene for the synthesis of GABA into the brain of a patient with Parkinson's disease. He was the first of a phase I clinical trial of this procedure. By 2007, several more Parkinson's patients had been treated with these injections with no harmful side effects and some improvement in their symptoms.

Several patients with an inherited lack of a functional gene needed to synthesize 11-cis-retinal — and thus destined to be blind — have had a useful level of vision temporarily restored in one eye injected with an AAV vector containing the gene (the other eye was the untreated control). Probably the fact that

the vector was injected directly into the eye and so not diluted throughout the body as an intravenous injection would be;

retinal cells rarely divide so the vector would not be lost. (The vector used had the genes needed for integration into the host cell's DNA removed so it could not be duplicated in S phase and, in dividing cells, would eventually disappear.)

Several children suffering from X-linked severe combined immunodeficiency have had their immune systems restored after retroviral gene therapy. [Link to discussion]

A few patients with hemophilia A have shown modest improvement when injected with their own cells that had earlier been harvested and transformed in vitro with a plasmid containing the factor VIII gene.

Several gene therapy agents — using adenoviral vectors — are in clinical trials and have shown some promise.

Among these:

a recombinant adenovirus encoding p53, a tumor-suppressor protein missing in many cancers

People with the rare disorder lipoprotein lipase deficiency are unable to process the globules (chylomicrons) of fat and protein that appear in the blood after a fat-containing meal because they lack functional copies of the gene encoding lipoprotein lipase. Intramuscular injection of an AAV vector containing the functional gene provides sufficient improvement, with apparent safety, that in October 2012, this agent (Glybera®) received approval for use in the European Union. It is the first gene therapy to receive such approval.